Outcome

The purpose of this call to add_theme_support() is to enable the Custom Background screen in the administrator's Appearance menu. It also serves as a declaration that the theme has properly implemented the body_class() and wp_head() template tags to provide feature compatibility.

When the administrator sets custom values for the theme, WordPress generates an extra style sheet in-line with the HTML headers, usually right before the end of the document's HEAD element. The extra style sheet overrides the background values from the theme's style sheet.

Note that setting the default-image parameter will instantly cause that value to become the effective Custom Background, whereas setting the default-color has no effect until the administrator visits the Custom Background page. To override this default behavior, you would have to provide a replacement for the _custom_background_cb() function.

Backwards Compatibility

To add backwards compatibility for older versions, use the following code: